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Permanent eclipse?

Hi there,

As I am writing my thesis right now my mind wanders off a lot. One of the things I was wondering about is what would happen if a planet has several moons, one of which moves stationary to the planet and sun. Would this cause a permanent eclipse? And in that case: what would be the influence of this 'shadow' if the planet is an earth like planet?

I am planning on using this concept in a D&D aventure if it proves to be viable.

I don't quite understand the scenario you have in mind. Do you mean that the orbital period of one of the moons is synchronized with the rotation of the planet and the orbital plane is such that there is a strip on the surface of the planet that never gets sunlight or only gets sunlight part of the day? Or do you mean that somehow the orbit of the moon is synched with the orbit of the planet around the sun so that there is a circular area on the surface of the planet that never gets sunlight at all? Or something else?

First, a Solar Eclipse is a localized thing. Only a very tiny portion of the surface of Earth ever sees any particular solar eclipse as total. In order to change that, you would need to make the "moon" considerably bigger than that "planet". This is what happens in a Lunar Eclipse (The Earth eclipses the Sun relative to the Moon, and its shadow covers the whole moon)

Second, to be "stationary" like that, you have to put the moon at the L1 point, or really, given how big the "moon" would have to be, you'd put the planet at the L2 point. Neither L1 nor L2 is stable in the long run though.

The only way I see that you could maybe make this work would be to put your planet at the L2 point of a gas giant, where it would be knocked loose rather readily by the influence of other planets or the giant's normal moons. If it somehow stayed in the L2 position, it would be frozen solid and completely uninhabitable.

So just call it magic. Go with a planetocentric model where the sun is small and goes around the planet.

To have something at the L-1 Lagrange point (directly between the sun and planet) that's large enough to cast a shadow, it would have to be very large and of negligible mass to be even reasonably stable. It wouldn't leave an area in perpetual darkness, though; as the planet rotated under it, the shadow would move, albeit in a predictable path.

However, if your goal is to have a body that leaves a particular area in darkness of similar size to a lunar eclipse here on Earth (100ish km diameter full dark and 6400ish km partial darkness), then the interposing body would have to possess quite an unusual orbital path (that is, it would effectively be a powered vessel of some sort). The area in darkness would basically just be what you'd get with perpetual night: not much in the way of plant life and fairly cold relative to the surrounding areas. Such as spot would probably wreak havoc with local weather and wildlife. However, there might be a thriving tourist industry of folks coming to dispose of items there ("Stick it where the sun don't shine" would be a pilgrimage of sorts rather than merely a rude suggestion). It would be a lot like the traditional underdark in the center, but with open skies.

In the Ringworld books, there are very large floating cities tethered in place (on the order of miles across, if I recall). The areas under these cities were in perpetual darkness, with similar effects to the above. A major difference was the presence of sewers and other things emptying down from the city above onto the landscape, leading to a whole culture of dark-adapted night people who disposed of the dead and other such things.

First, a Solar Eclipse is a localized thing. Only a very tiny portion of the surface of Earth ever sees any particular solar eclipse as total. In order to change that, you would need to make the "moon" considerably bigger than that "planet". This is what happens in a Lunar Eclipse (The Earth eclipses the Sun relative to the Moon, and its shadow covers the whole moon)

Second, to be "stationary" like that, you have to put the moon at the L1 point, or really, given how big the "moon" would have to be, you'd put the planet at the L2 point. Neither L1 nor L2 is stable in the long run though.

The only way I see that you could maybe make this work would be to put your planet at the L2 point of a gas giant, where it would be knocked loose rather readily by the influence of other planets or the giant's normal moons. If it somehow stayed in the L2 position, it would be frozen solid and completely uninhabitable.

So just call it magic. Go with a planetocentric model where the sun is small and goes around the planet.

There's no denying that it would be frozen solid but you could also make your planet a moon in orbit around a large gas giant "think pitch black" May be reversed the scenario the planet stays in the dark most of the time except for a month out of the year

Course if it was a gas giant there would be some leakage of light so wouldn't be perfectly black course you could use some artistic license and do some magic, you might even have to do some magic with the orbit